If the original GB-E went down, they would have reassigned the aircraft ID to a another machine, otherwise you'd run out of letters. I believe the picture of the row of 105 Sqdn aircraft was taken in December 1942.

Yep, nothing unusual. The individual code letter, in this case 'E', would be issued to another airframe in the event one was lost, transferred to another squadron, or non-operational for whatever reason. As an example, it was not uncommon for an aircraft to be on a squadron as, say, 'E', and then be moved to an MU for repair, modification or whatever, only to return to the same squadron, and then be marked as, for example, 'G'.
This shows up often in research, where an aircraft is found on squadron records wearing two, or even three, different code letters.

Date: 31-OCT-1944
Time: 12:30 LT
Type: de Havilland Mosquito FB.Mk.VI
Owner/operator: 487 (NZ) Sqn RAF
Registration: PZ164
C/n / msn:
Fatalities: Fatalities: 0 / Occupants: 2
Other fatalities: 0
Airplane damage: Written off (damaged beyond repair)
Location: Harplinge, near, 11 km NW Halmstad (Halland) - Sweden
Phase: Combat
Nature: Military
Departure airport: Swanton Morley
Destination airport:
Narrative:
Damaged by bomb blast, Aarhus (Denmark). Emergency belly landing in Sweden. Aircraft burned out.
"We took off from Thorney Island at 0700 hours on 1 November 1944 in a Mosquito aircraft. We landed at Swanton Morley to refuel, taking off on our mission which was to bomb the Gestapo Headquarters at Aarhus, Denmark at 0930 hours.
"We had released our bombs and as we passed over the target at 100 feet the bombs which had been released by a preceding aircraft exploded. Our aircraft was damaged and we were forced to feather the starboard propeller. We therefore followed briefing instructions and orders from the formation leaser and flew to Sweden.
"We landed in a field near the village of Harplinge, near Halmstad, Sweden and burned the aircraft. A policeman held us until the Army authorities arrived. We were then taken to Halmstadt and billeted at the Grand Hotel under armed guard. An interrogation was attempted but was not pressed.
"On 2 November we were taken to Falun via Gothenburg. At Gothenburg we escaped from our escort with the idea of reaching the British Consul. (We had been given to understand at the Squadron that if we could reach the British Consul in a neutral country we would be repatriated immediately.
"Wing Commander Thomas"I ran from the brightly lighted railway station at Gothenburg into a dark square. Running accross the square I jumped over a low chain. In mid air I realised that I had jumped out over a canal but it was too late to do anything about it. I fell 15 feet into the water and was pulled out and recaptured by the escort, police and civilians.
"F/L Humphrey-Baker"I had been running just behind W/C Thomas and when he disappeared I realised his mistake and turned to one side just in time. I continued on and reached the British Consul, who immediately turned me over to the police, where I rejoined W/C Thomas. We were then taken to Falun together.
"On 3 November we were billeted at the Solliden Pensionat Hotel and we were kept there until 14 November. During that time W/C Thomas ferried a PRU Mosquito from Malmo to Linkaping at the request of the Air Attache. On 14 November we went to Stockholm to be repatriated. On 23 November we were sent to the UK by air."

Yep, nothing unusual. The individual code letter, in this case 'E', would be issued to another airframe in the event one was lost, transferred to another squadron, or non-operational for whatever reason. As an example, it was not uncommon for an aircraft to be on a squadron as, say, 'E', and then be moved to an MU for repair, modification or whatever, only to return to the same squadron, and then be marked as, for example, 'G'.
This shows up often in research, where an aircraft is found on squadron records wearing two, or even three, different code letters.